1. Field of the Invention
The invention relates to a method of removal of polycyclic aromatic hydrocarbons (PAH) from a liquid, for example waste water from a coking process, by precoat vacuum filtration using precoat material comprising a finely divided organic material. The invention also relates to a method of testing a finely divided organic material for its suitability for use as a precoat material in precoat vacuum filtration, e.g. for removal of PAH.
Precoat vacuum filtration is a known filtering process which in essence comprises the steps of
(i) applying to the exterior of the filtering surface of a vacuum filter drum of a rotary vacuum filter a particulate precoat comprising finely divided organic particles,
(ii) filtering said liquid through said filter drum with said precoat thereon, by partially immersing the rotating drum in the liquid and applying reduced pressure at the inside of the drum,
(iii) continuously or intermittently removing the surface layer of the precoat at the exterior of the precoat thereby to present, to subsequently filtered liquid, fresh particles of the precoat at the exterior of the precoat.
Normally the drum is rotated with its axis horizontal. Conventional rotary vacuum filter apparatus may be used, the details of which are known to the expert in this field and need not be repeated here.
Precoat vacuum filtration has the effect of removing solid particles carrying PAH from the liquid and also of removing a second liquid phase containing PAH which may be present.
Precoat vacuum filtration is fully described and explained in "Filtration for removal of polycyclic aromatic hydrocarbons from coke oven waste water" by A. van Hoorn published in "Fachberichte Huttenpraxis Metallweiterverarbeitung", Vol. 22, No. 10, 1984 pages 1088-1092 the contents of which are herein incorporated by reference. This article, written by one of the present inventors, states that it is possible to remove PAH out of waste water from a coking plant by means of precoat vacuum filtration using wood flour as precoat material. As far as is known, this is the only method for removing PAH with an acceptable yield. It has now been found that the choice of the precoat material is critical to this method.
A problem with the known method is that filter performance and filtration properties can vary considerably from batch to batch of wood flour. Attempts to relate these differences to the wood flour grain size distribution and with the results of other known methods of analysis resulted in failure.